1. Field of the Invention
The present invention relates to a ball screw device and a steering system.
2. Description of the Related Art
As a conventional electric power steering system (EPS), an EPS is known that uses a ball screw device to convert rotational torque of a motor into a linear motion of a rack shaft, thereby assisting a driver in steering operation.
In such a ball screw device, a ball screw nut that is rotated by a motor is screwed onto a rack shaft via balls. In conjunction with rotation of the ball screw nut, the balls roll while rotating in the same direction between the ball screw nut and the rack shaft. If adjacent balls come into contact with each other during rolling of the balls, sliding resistance is generated between the adjacent balls, which causes the rotational torque of the ball screw device to fluctuate. In view of this, a ball screw device of a steering system described in Japanese Patent Application Publication No. 2009-257369 (JP 2009-257369 A) is provided with a retainer (cage) having retainer grooves that retain balls in a rollable manner in order to prevent adjacent balls from coming into contact with each other. Between adjacent balls in the circumferential direction, separating portions each of which is a portion between adjacent retainer grooves of the retainer are provided, whereby the adjacent balls in the circumferential direction are prevented from coming into contact with each other.
In the retainer of JP 2009-257369 A, the stiffness (deflection stiffness in particular) of each separating portion varies with the position in the axial direction. In other words, the stiffness of both end portions of the separating portion in the axial direction is higher than the stiffness of a central portion that is positioned between both end portions of the separating portion.
In this regard, while the stiffness of each separating portion is preferably low from the viewpoint of easy insertion when the balls are inserted between the retainer and the nut, the stiffness of the separating portion is preferably high in order for the retainer to properly retain the balls. However, when the stiffness of the central portion of each separating portion is suitable, the stiffness of the separating portion is high in the end portions, which makes the insertion of the balls difficult. When the stiffness of the end portions of the separating portion is suitable, the stiffness of the separating portion is low in the central portion, and thus the balls may fail to be properly retained and may fall off. When the clearance between the balls and the retainer is reduced so as to prevent the balls from falling off, a situation may occur in which the balls roll while being caught by separating portions more strongly at the end portions of the separating portions than at the central portions of the separating portions due to dimensional variation of the separating portions. This may increase the friction between the balls and the retainer. Thus, a retainer is desired that has separating portions having more uniform stiffness in which the difference between the stiffness of end portions of each separating portion and the stiffness of a central portion of the separating portion is small.